Digital BW, The Print

Over time, I've been aware of posts here regarding banding in prints from various 
machines.  I didn't pay a lot of attention because I wasn't having the problem.  Now I am 
having the problem.

I've done a search in this forum and found that the diagnoses are all over the place, 
depending on specific machine model, inks, cartridges, paper, etc.  If anyone can provide 
some guidance I'd be most appreciative.

Here are some details:
Epson R2400
K7 inks in Superjet USA CISS (just recently installed as an experiment)
Epson Enhanced Matte paper
QTR settings standard with 2800 dpi and unidirectional printing

Test target: Jon Cone's proofofpiezography.psd

All tones are clearly visible, down to and including 1%, on the test strips.

Problem: faint banding becomes visible at the 25% level and below.

Any thoughts on how to make progress toward a diagnosis?

Harry